1. Scope of the Invention
The present invention is in the field of analog-to-digital conversion techniques. More specifically, the present invention is an electronic circuit for correcting thermal codes suffering from bubble errors.
2. Description of the Prior Art
A flash-type analog-to-digital converter (hereafter "ADC"), possesses faster ways to achieve analog-to-digital conversion than counting ADCs, successive-approximation ADCs, or ratiometric ADCs. The flash-type ADC is also described as a "parallel-comparator ADC" for the reason that several comparators are used to make parallel comparisons with a number of different reference voltages respectively. In recent years, because the fully parallel ADC makes great demands for layout area and power consumption, subranging two-step conversion architectures, which divide one comparison process into a coarse decision cycle and a fine decision cycle, have been developed. See B. S. Song et al., "A 10-b 15-MHz CMOS RECYCLING TWO-STEP A/D CONVERTER," IEEE Journal of Solid-State Circuits, vol.25, No.6, December 1990, pp.1328-1338. The two-step ADC trades speed for smaller layout area and lower power consumption, and it also has the potential for high-resolution applications.
Normally, the outputs of comparators in the flash-type ADC form a thermal code. Then, a priority encoder is used to convert the thermal code into a binary code or a binary-coded-decimal (BCD) code for further processing. The thermal code must satisfy high resolution requirements, but this makes it susceptible to process variation or noise interference, and thus it suffers from bubble errors. The bubble is an output code quite deviant from the input value which occurs sporadically. For example, a 10-bit two-step ADC with a reference range of 2 V needs to differentiate about 2 mV (2 V/2.sup.10) prior to the generation of the thermal code. Such tiny voltage difference makes the outputs of comparators quite sensitive to process variation and noise interference. For instance, noise may be induced and so interfere with the outputs of the comparators while the comparators are activated simultaneously.
To overcome the aforementioned problem, Yuji Gendai et al. have proposed "AN 8 b 500 MHz ADC" in 1991 IEEE International Solid State Circuits Conference, which utilizes a differentiator to detect a transient point in a thermal code, and two encoders to average the detected results. However, while Gendai's circuit may be quite suited to dealing with those bubble errors whose occurrence is predictable, it will be unable to deal with bubble errors that occur randomly.